The present invention relates to electrophotographic recording apparatus such as copiers and laser printers and more particularly to an electrophotographic recording apparatus suitable for color image printing.
In the past, an electrophotographic recording apparatus such as a copier and a laser printer produces a print through an electrophotographic process in which an electrostatic latent image is formed on a primary recording medium such as a photosensitive drum, the latent image is converted into a toner image through development using a developer made of toner or a mixture of toner and carrier, and the toner image is transferred to a secondary recording medium such as a paper or an OHP sheet and is then fixed.
The color electrophotographic recording apparatus for producing a two-color or polychromatic image or a full color image by synthesizing (super-imposing) toner images of different colors formed on the basis of signals obtained by color-separating a copy paper or signals representative of information about components of color of a color image, which signals are obtained by decomposing the color and used to form the color image, uses a photosensitive member, for example, a photosensitive drum or a photosensitive belt as a primary recording medium and is classified into the type wherein toner images of different colors are formed on the photosensitive member in overlapping fashion, the type wherein toner images of different colors formed sequentially on a single photosensitive member are transferred sequentially in overlapping fashion to a secondary recording medium wound round a transfer drum so as to be synthesized, the type wherein toner images of different colors formed sequentially on a single photosensitive member are transferred in overlapping fashion to an intermediate transfer member so as to be synthesized and thereafter transferred to a secondary recording member, and the type wherein toner image forming process means such as photosensitive members and developing units are provided by the same number as that of color separations and toner images of different colors formed on the photosensitive members are transferred sequentially in overlapping fashion to a secondary recording medium so as to be synthesized.
Of them, the type wherein toner images of different colors are formed on the photosensitive member are transferred to the intermediate transfer member in overlapping fashion to provide a color toner image and thus obtained color toner image is transferred to the secondary recording medium such as an OHP sheet is advantageous in that the apparatus can be compact because means for transferring toner images of different colors in overlapping fashion with high accuracies can be constructed relatively easily.
Electrophotographic techniques as above are disclosed in Japanese Patent Application Laid-open Nos. JP-A-1-502062, JP-A-59-50474, JP-A-59-125765, JP-A-1-288870, and JP-A-2-212870.
For example, in a color copier disclosed in the aforementioned Japanese Patent Application Laid-open No. JP-A-1-502062, toner images of a plurality of different colors sequentially formed on a photosensitive belt are transferred in overlapping fashion to an intermediate transfer drum to produce a color toner image and thus produced color toner image is transferred to a recording paper and then fixed; and in a recording apparatus disclosed in the aforementioned Japanese Patent Application Laid-open No. JP-A-59-50474, a toner image formed on a photosensitive drum is transferred electrostatically to an adhesive intermediate transfer member and thereafter press fitted to a recording paper so as to transferred and fixed thereto through heating.
In the color electrophotographic recording apparatus as above, the transfer of toner image is carried out plural times and it is necessary that raising transfer efficiency in each transfer process be contrived. If force necessary for the toner image to be adhered to the intermediate transfer member is increased in order to increase the efficiency of transfer of toner image from the photosensitive member to the intermediate transfer member, then the toner image will hardly separate from the intermediate transfer member, making it difficult to raise the efficiency of transfer of the toner image from the intermediate transfer member to the secondary recording medium.
To increase the transfer efficiency in both the transfer processings, potential gradient at the two transfer sections has to be adjusted but when potential adjustment are carried out locally within the same intermediate transfer member at a time, mutual interference takes place to make achievement of optimum adjustments of the two transfer sections difficult and the construction complicated, thus causing a bottleneck in designing a compact apparatus.
Further, if upon transfer of a toner image from the intermediate transfer member to the secondary recording medium the transfer charge remains on the secondary recording medium then the secondary recording medium will be adsorbed electrostatically to the intermediate transfer member and disadvantageously wound thereround. Generally, discharge means is adapted to prevent this problem but it is affected by the potential of the intermediate transfer member, making it difficult to reduce the electrostatic adsorptive force.
In the apparatus which employs the heating transfer method with a view of improving the efficiency of transfer from the intermediate transfer member to the secondary recording medium, the intermediate transfer member is also heated and heat generated therein is transferred to the photosensitive member and therefore both the members are required to be heat-resisting. Especially, preparation of a heat-resisting photosensitive member faces much difficulties and is unpractical.
In addition, facilitation of developing unit exchange working and toner replenishment working is not taken into account and besides facilitation of working of processing a jam which would take place in the paper feed mechanism for conveying the secondary recording medium and maintenance working for various rollers and fixing unit is not taken into full consideration.